There has long been a demand for the production of multi-channel audio signals from two-channel stereo audio signals, and numerous audio devices have such a function. However, it is known that such devices involve a feeling of reversed phase and a feeling of disharmony during playback.
In the conventional production of multi-channel signals from two-channel signals, especially signals known as “surround” or the like that are played back from the area extending from the sides to the rear of the listener, or signal OSL and OSR, which are signals that are not localized in the area extending from the sides to the rear of the listener, [a method in which] the difference between the input stereo audio signals INL and INR is calculated as shown in FIG. 1 an the following “Equation 1” is calculated and played back is generally used.OSL=INL−INR OSR=INR−INL  Equation (1)
In this case, since OSL and OSR are mutually reversed phases, it is quite natural that the listener experiences a feeling of reversed phase during playback. Specifically, FIGS. 8 and 9 show examples of the waveforms and frequency characteristics of the stereo audio signals INL and INR that constitute the input signals. Surround signals such as those shown in FIGS. 10 and 11 are produced by subjecting such stereo audio signals INL and INR to the processing shown in FIG. 1.
As is clear from this FIG. 10, if surround signals are merely produced from the difference between the left and right stereo audio signals, the left and right surround signals OSL and OSR have reversed phases. Furthermore, as is shown in FIG. 10, the left and right signals have the same amplitude but reversed phases; accordingly, the correlation is strong, and since the signals are completely different from the stereo audio signals that are the production source, the feeling of disharmony during playback is not eliminated.
Furthermore, as is shown by the frequency characteristics in FIG. 9, the left and right input signals both have common signal components in the vicinity of 4.5 kHz, and these components are a cause of the feeling of disharmony. In the surround signals produced from the difference between such input signals, the left and right signals are constructed from the same frequency components as shown in FIG. 11, so that the correlation of both signals is extremely strong, and there is a strong unnatural impression.
Accordingly, there have been proposals to reduce the correlation between surround signals, and thus eliminate the reverse phase feeling and feeling of disharmony experienced by the listener. However, conventional techniques of this type of not go beyond simple phase manipulation, amplitude manipulation and the like; there have been no proposals of essential correlation elimination processing in the production of surround signals.
Furthermore, in quasi-stereo processing and the like, there have been widely used correlation elimination methods, e.g., correlation elimination processing using comb filters or the like. However, since such phase elimination processing is performed on signals obtained by “Equation 1”, i.e., signals that have the same amplitude by reversed phases, the elimination of a reversed phase feeling and a feeling of disharmony has not yet been achieved.
The present invention was devised in order to solve the problems encountered in the prior art; it is an object of the present invention to provide an audio device that eliminates a reversed phase feeling and feeling of disharmony by performing correlation elimination processing that introduces an adaptive signal processing technique for the production of surround signals.